Production of graphite fibers

ABSTRACT

Graphite fibers having superior strength and modulus of elasticity can be obtained by twisting carbon fibers at a specified twisting rate, and then graphitizing the twisted yarn.

I United States Patent [151 3,702,054

Araki et al. 1 Nov. 7, 1972 [541 PRODUCTION OF GRAPHITE FIBERS [721 Inventors: Tadashi Araki; Kiro Asano; Jun [56] Rem-anus Cited Yamada, all of Tokyo, Japan UNITED STATES PATENTS 1 Assigneer Kufeha Kagaku sy Kabushiki 3,378,999 4/1968 Roberts et al. ..57/139 Kalshfl, Tokyo-t0, Japan 3,379,000 4/1968 Webber et al. ..57/ I 39 22 F! d2 I 27 1971 3,626,041 l2/l97l Fields et al ..57/l53 X l 1 y 3,648,452 3/1972 Young ..57/l64 211 App]. No.: 166,448

Primary ExaminerJohn Petrakes [30] Foreign Application Priority Data Attorney-E wenderoth et July 28, 1970 Japan ..45/65556 [57] ABSTRACT Graphite fibers having superior strength and modulus [52] "57/157 57/157 57/140 R of elasticity can be obtained by twisting carbon fibers [51] hit- Cl. at a specified twisting rate, and then g p i i g the [58] Field of Search .....57/140 R, 140 DY, 153, 164, twisted yam 57/156,157 S, 157 R, 157 TS, 34 HS; 28/72 HR 3 Claims, 5 Drawing Figures ANGLE CONSTITUTED BY DIRECTION OF FIBER AXIS AND DIRECTION OF C-AXIS OF CARBON CRYSTAL PATENT'EDunv 1 m2 SHEET 1 III 3 FIG.

0 20 40 I 60 8IO9O ANGLE CONSTITUTED BY DIRECTION OF FIBER AXIS AND DIRECTION OF C-AXIS OF 'CARBON CRYSTAL TADASHI ARAK-I, KIRO' ASANO 'JUN YAMADA AND INVENTOR S ATTORNEY S P'ATENTEBnuv 11972 3.702.054

sum 2 or s TADASHI ARAKI KIRO ASANO AND JUN YQVENTOR. BY J) @mda pmurznm 1 m2 3.702.054

SHEET 3 OF 3 TADASHI ARAKI, KIRO ASANO AND JUN YAMADA INVENTOR.

PRODUCTION OF GRAPHITE FIBERS This invention relates to the production of graphite fibers, more particularly, to a method of producing graphite fibers of improved modulus of elasticity and strength as well as having various polygonal cross-sections.

It is an object of the present invention to provide graphite fibers having different cross-sectional shape of various polygons.

It is another object of the present invention to provide graphite fibers having the abovementioned particular cross-sectional shape, which, when twisted, are intimately and compactly twisted with each other so as to reduce the concentration of stress in the twisted fibers.

It is another object of the present invention to provide graphite fibers having an improved high modulus of elasticity and/or high mechanical strength necessary for use as reinforcement for composite materials.

It is still another object of the present invention to provide a method for producing graphite fibers having the abovementioned particular cross-sectional shape and improved properties.

The nature, principle, and the abovementioned ob jects of the present invention will become more apparent and clearly understood from the following detailed description thereof when read in connection with the accompanying drawing.

In the drawing:

FIG. 1 is a graphical representation showing crystal orientation of graphite fibers obtained under different conditions;

FIG. 2 is an X-ray diffraction photograph of the graphite fiber according to the present invention;

FIGS. 3 and 5, respectively, are microphotographs showing the cross-section of the graphite fibers according to the present invention; and

FIG. 4 is a microphotograph showing the cross-section of graphite fibers obtained by the ordinary method.

In most cases, the cross-sectional shape of the heretofore known carbon fibers is determined by the manufacturing process of the raw material. Most common shapes are circles, ovals, cocoons, and stars. When carbon fibers of such cross-sectional shape are bundled and twisted into yarn (or thread), it is not possible to reduce space created among each of the fibers below a certain limit. In the twisted yarns obtained from ordinary carbon fibers, because of high rigidity of the carbon fibers, the space or porosity in' the twisted yarns reaches from 40 to 70 percent, or even higher, in a free state of no tension or compression being imparted thereto.

Further, as the carbon fibers are each in contact state mainly at various contact points, it is readily assumed that, when a pressure is imparted to the twisted yarns in the direction vertical thereto, there takes place a concentration of stress onto the contact points or tangential points of the fibers constituting the twisted yarns or threads. In the materials having high a Youngs modulus of elasticity such as carbon fibers, such concentration of stress is undesirable.

In general, there have already been presented many a report as to plastic deformation of carbon fibers by stretching the same in the axial direction thereof at a high temperature is an inactive atmosphere. However, there has been not a single report about how to change the cross-sectional shapes of the material like carbon fibers having extremely thin diameters and showing its plastic deformation (or flow) only at an extremely high temperature without impairing its mechanical characteristics since such an objective has been considered extremely difficult.

The characteristic feature of the present invention resides in having solved such difficult problems as mentioned above. That is, by treating carbon fibers of circular cross-section into a twisted yarn or thread under particular conditions, it has been found that the stress imparted to the fibers in the axial direction thereof at a high temperature is converted to a force which compresses the fibers in the direction vertical to the axis thereof. This is due to the pressure in which the fibers change from a state of being close to a state of line contact thereof to another state of being close to a state of plane contact thereof, or the fibers as a whole approximate a state of compactness from coarseness. Moreover, when the cross-sectional shape of the raw material carbon fibers is, in general, circular or oval which is very close to a circle, the cross-section of the fibers after the deformation treatment according to the present invention converts the fibers into different polygonal cross-sections.

To explain the above phenomenon in more detail, stress to be imparted to the carbon fibers at a temperature region of 2,000 C. and above is not only a negative pressure acting in the lengthwise direction of the fibers, but also a positive pressure along the cross-section of the twisted yarn. In this case, the positive pressure along the cross-section of the twisted yarn changes the cross-section of the fibers into different polygonal cross-section by the mutual compressive action of the fibers with the consequence that the bundle fibers as a whole are brought to a state, wherein each and every fiber is mutually and compactly bundled due to the deformed cross-sections of the individual filaments. This fact signifies that the individual fiber is not in point-contact with others along the lengthwise direction of the fiber as in the ordinary twisted yarn, but constitutes a linear or planar contact with each other.

Moreover, the positive pressure along the cross-section of the fibers as described above increases the stretching force in the lengthwise direction of the twisted yarn as a compressive force to the individual fiber. Accordingly, the fibers which are in the state of being readily subjected to plastic deformation are further elongated in their lengthwise direction by these two kinds of forces, at which time there takes place growth and orientation of the graphite crystallites resulting in remarkable increase in the modulus of elasticity during the stretching operation. This increase in the modulus of elasticity may possibly exceed 700 depending on the elongation conditions. At the same time, it may also be possible to cause the degree of crystal orientation to exceed percent. Yet, in spite of such remarkable increase in the modulus of elasticity and high orientation of the crystallites, the carbon fibers are still greatly increased rather than decreased in the mechanical strength. The rate of increase in mechanical strength may reach almost 400 percent.

There have still been existing differences between the theoreticalv values and the measured values in the modulus of elasticity and mechanical strength in the heretofore known carbon fibers as follows:

As seen from the above, while a comparatively high rate of measured value with respect to the theoretical value has been accomplished in the modulus of elasticity, the measuredvalue in the mechanical strength is only about one-fifth to one-tenth of that of the theoretical value, even when the measured value of 200 T/cm in graphite whisker is compared. It is therefore assumed the mechanical strength of the carbon fibers and graphite fibers is governed by the defective portions existing'in the amorphous portion, surface portion as well as the internal portion of such fibers rather than in the crystalline portion thereof. I

It is considered that this increase in mechanical strength would have been achieved by greatly reducing those fine voids (or pores)v existing in the. fibers and those defects existing in the surface of the fibers through compressive force exerted at the time of deformation due to compression among the fibers in a twisted state along the lengthwise as well as axial direction thereof, which can hardly be achieved by the force exerted in the axial direction of the fibers alone. Thus, the increase in the modulus of elasticity without an accompanying lowering in mechanical strength would enable the greatest shortcoming in the heretofore known graphite fibers to be overcome, thereby representing a great industrial achievement in this field.

Since the graphitic fibers to be obtained by the present invention possess the above-described characters, it is possible to manufacture graphite fibers of superior quality such as, for example, fibers having mechanical strength of 37 T/cm and a modulus of 5 elasticity of 4,600 T/cm the values of which exceed those of boron fibers (mechanical strength of about'32 T/cm and modulus of elasticity of about 4,300 T/cm The boron fibers have so far been regarded as having both mechanical strength and modulus of elasticity to the highest degree and in the most balanced state as the reinforcement material for composite materials.

Furthermore, the afore-described ideas of polygonal cross-section and compact filling of fibers due to such new cross-sectional shape greatly contribute to the improvement in the characteristic of the graphite fibers as the composite material. In particular, it is possible to raise the mixing ratio of the fibers as the reinforcement to more than 80 percent.

In order to carry out the treatment according to the present invention in a most quick and efficient manner, the temperature for the carbon fibers to exhibit their plastic deformation is required to be higher than 2,000 C., or more preferably between 2,500 C. and 3,500 C. or so. For the atmosphere in this heat-treatment, an inactive gas atmosphere such as argon, helium, etc. is used. In particular, when the fibers possess, a high modulus of'elasticity such as less than 0.9 percent or so of elongation at breakage, it is desirable that a temperature of more than 2,900 C. and a sufficient time for the heat-treatment be given so as not to damage the fibers. Ordinarily, it is preferable to impart to the fibers to be treated such a temperature, twisting condition, and

stress to enable a sufficient plastic deformation thereto within about 1 minute of time. This heat-treatment may be carried out either in a single step or in a divided twostep process.

The raw material carbon fibers tobe subjected to the treatment in accordance with the present invention may be either carbonaceous or graphitic, but should be circular in the cross-section, possess a tensile strength of more than 7 T/cm, and contain carbon of more than percent by weight so as not to bring about lowering of strength during the high temperature treatment. The average fiber length may be between 75 mm and mm to ,sufficiently attain this purpose. While the fiber length should of course be determined in terms of the number of fibers in the whole twisted yarn as well as the twisting conditions, the above specified figures are the shortest average lengths when the other conditions are in the most desirable ranges.

Carbon fibers satisfying the above mentioned conditions may be useful from whatever material and in whatever method they are produced. However, from the standpoint of the particular restriction in the present invention that the cross-sectional shape of the fibers should be circular, those carbon fibers obtained from pitch as the raw material are most preferable.

In the case of the carbon fibers having notoriously irregular cross-sectional shapes such as star-like shapes, or the like, there tends to occur preferentially a destructive phenomenon due to the concentration of stresses, which does not make for an increase in the mechanical strength of the fibers. On the other hand, carbon fibers producedfrom raw material pitch at a high spinning speed and with a high stretching ratio possess polygonal cross-sections, the shape of which enables the carbon fibers to be intimately and compactly twisted with each other, so that they are most suited for producing graphite fibers having a high modulus of elasticity and high mechanical strength according to the present invention.

In order that the stress imparted in the axial direction. of the twisted yarn may be converted to a compressive force in the direction perpendicular to the axis of the fiber, according to the present invention, there is required a process of giving proper twisting to a certain appropriate numbers of carbon filaments, and, if need be, a further twisting of such twisted yarns is necessary to give them double twisting.

If the twisting is not done properly, a sufficient conversion of the axial stress into a compressive force perpendicular to the fiber axis cannot be achieved, and the advantageous effects effect of the present invention cannot be attained. Although there have been considerable difficulties in arranging various factors such as the first twisting, second or further twisting the, number of filaments to be twisted, and so forth in concise terms, the present inventors have found out that, when the ratio between the apparent rupture strength of the twisted yarn the and average strength of each filament constituting the twisted yarn (i.e. conversion efficiency) is in the range of from 0.03 to 0.5, the

present invention can be effectively carried out. 2

When the constituent filaments have their elongation at a breaking point above 0.9 percent, the deforming not intended to narrow the scope of protection as afforded by the appended claims.

EXAMPLE 1 treatment can be carried out at a practical treating tem- 5 perature and time without damaging the twisted fila- Pitch containing 96.2 percent by weight of carbon, ment within the above specified range of the converhaving mean molecular weight of 870, and a softening sion efficiency. However, when the elongation at the point of 178 C. was obtained from the cracking of breaking point is below 0.9 percent, the Conversion efpetroleum naphtha at a high temperature. ficiency is found to be appropriate between 0.07 and The pitch was melt-spun into fibers at various 0.2. Incidentally, the rupture strength of the twisted spinning speeds and stretch ratios as shown in the folyam should not be lower than 0.5 T/cm (Fiber length lowing Table 1, and the thus obtained pitch fibers were of the test sample for measuring the rupture Strength of heat-treated in air starting from a room temperature up the twisted yarn is selected to be one half of the average I 5 o 250 at a mp r r ri r f -8 -Imin, fiber length of the constituent filament, or 300mm, after which the heat-treated (infusibilized) fibers were which ever is shorter). subjected to a carbonization treatment for 30 minutes When the twisted yarn of the above-described conat 1,000 C. in anitrogen atmosphere. struction is stretched in the axial direction of the fiber The Carbon fibers Obtained in the abO e mentioned at a high temperature, there takes place a plastic deformanner had. in average, a mechanical s reng h of 7 to 8 mation of the yarn due to the converted compressive a o u o l ici y of 550 to 650 7 an force in proportion to the degree of stretching of the elongation of 1.3 percent, a carbon content of 99 pertwisted yarn. Accordingly, there should exist a and acircular fi -se minimum stretching stress for this compression defor- The carbon fibers of the above-described Properties mation, the required range of which sho ld at least b were processed into the following two conditions, and 0.5 T/cm. Below 0.5 T/cm the purpose of the present graphitized y indil'eCtly heat'treating them at a invention cannot be attained. The figure given above p rature range Of from 2,750 to 2,800 C. in a graphite slightly varies depending on the figure region at which tube which has been heated y high frequency inducthe yarn is treated. When this figure is converted to a lion and coma-ins therein argon stretch ratio of about 3,000 C., it reaches more than 1.4 times as great as that of the twisted yarn in case, for Processed Condmons f Carbon Flbers example, the length of the twisted yarn is shorter than a. Filaments not twisted the average length of each filament constituting such (Sample Nos. 1, 2. 3, 4, and4ain Table l) twisted yarn as in filament yarn, wherein the influence b. 500 carbon filaments having average length of due to mutual sliding of the filaments is negligible. 75mm were subjected to first twisting (8.2 twists/inch) Also, when the average fiber length is shorter than the to give the twisted yarn a strength of about 1.8 T/cm, twisted yarn length the, apparent stretch ratio of ordi- (ratio of strength 0.24) then three sets of the twisted nary staple yarn becomes larger than the effective yarns were subjected to further twisting (second twiststretch ratio with the consequence that a stretching of ing) at a rate of 5.8 twists/inch. more than 1.6 times that of the twisted yarn length is (Sample NO- 5 n Table I required. Of these test samples, Nos 2 and 4 fiber samples were The fibers which have been compression-deformed heat-treated by fixing one end of the total fiber length under the above-mentioned conditions are intimately of 150 mm, and hanging a weight at the other end and compactly twisted as mentioned already, being thereof so as to impart a stress of 0.7 T/cm to the fibers close to the plane contact, with the result that rate of and to result in elongation of about 15 percent. In the mixing of such twisted yarn as a reinforcement in formcase of No. 5 sample fibers (twisted) and No. 4a sample ing a composite material can be increased at an exfibers (non-twisted), the heat-treatment was conducted tremely high degree. This excellent property also imunder the same conditions as in sample Nos. 2 and 4 parts extremely superior capability in the properties of until about 80 percent of elongation is reached under the resulting composite materials, in which point the an imparted stress of 1.3 T/cm superior effects of the present invention are realized. Graphite fibers obtained in this manner indicated re- In order to enable those skilled in the art to readily markable differences as shown in Table 2 below, from practice the present invention, the following preferred which the superiority of the method according to the examples are given. It should however be understood present invention as represented by No. 5 sample fibers that these examples are illustrative only, andthey are can be clearly recognized.

TABLE 1 Spinning Graphitization Stress Sample Temp. Speed imparted 0. Method 0.) (m./min.) lam?) 1 Melt extrusion" 270 250 2 do 270 250 0.7 310 2, 000 310 2, 000 0. 310 2,000 1. 310 2,000 2 TABLE 2 Modulus of 1 Cross- Strength elasticity Elongation sectional Sample No. (T/cm!) (T/cm (percent) shape 3.7 250 1 5 Circular. 2.1 270 1 D0. 6.5 430 l Do. 17.0 2, 000 0 9 Do. 20.0 3,600 0 8 Polygonal. 18. 0 2, 300 0 8 Circular.

' The Nos. 3, and 5 sample fibers were measured for their crystal orientation through X-ray diffraction, the results of which are shown in FIG. 1.

While it is considered adifficult problem to deter- I mine on what ground the remarkable difierences took ficulty (vide: curve 1 in FIG. 1), or, fine pores or voidsv tend to develop in an enormousamount among the crystals along with crystal growth. This phonomenon might possibly be related to an increase in diameter of the filament of about to 20 percent or so, which is' observable during the heat-treatment.

In the case of sample fibers Nos. 3,4 and 5 obtained by high spinning speed and draft ratio, there is less possibility of relatively large defects occurring on-the filament surface. This shows that there is much more possibility. of most of the fibers obtained by slow spinning speed and low draft ratio having defects that cannot be eliminated by the change in the fiber structure due to graphitization. This fact also appears to be the cause of the differences in properties between the graphite fibers of sample Nos. 1 and 2 and the graphite fibers of sample Nos. 3 and 4.

Furthermore, the twisted graphite fibers obtained by the method of present invention (Sample No. 5) are recognized to have the following three major differences from the graphite fibers obtained by ordinary graphitization treatment.

In the first place, the mechanical strength of the fibers are improved. This improvement in strength is considered due to the reduction on the defects existing in the fiber surface and fine pores present in the interior of the fiber itself as a result of the compressive force exerted among the fibers. This improvement can also be inferred from the difference in specific gravity of more than 0.2 g/cm among Sample Nos. 4, 4a and 5.

In the second place, the method according to the present invention makes it possible to maintain a large elongation of the graphite fibers without losing the mechanical strength (vide: curve 5 in FIG. 1 In the ordinary graphite fibers, when the fibers are oriented into crystals that such a modulus of elasticity of more than 3,500 T/cm is imparted thereto, it is usually difficult to obtain a mechanical strength higher than 22 T/cm. If obtainable, the elongation becomes only 0.5 percent or so. The improvement in this respect results in particularly favorable pattern in the stress-strain curve of machine parts made of composite materials reinforced with the graphite fibers, which means that the reinforced composite materials can be used in a variety of machines because of their increased strength.

In the third place, the fibers are mutually compressed to" deform their cross-sectional shape from the initial circular shape to a hexagonal or other polygonal shapes, whereby the bundle of the fibers are intimately and compactly combined as shown in FIG. 3. For ex ample, in sample No. 5, the degree of compactness of the bundle of the fibers reaches 88%, while the graphite fibers obtained by the ordinary method has a'compactness of only 62 percent. This fact is significant in that, when the fibers are used for composite materials, they can be mixed into the material to an extremely high degree. For example, in the case of the sample fibers Nos. 4, 4a, and 5, it is very difiicult to mix the fibers of sample Nos. 4 and 4a at a rate of more than 65 percent and above with respect to the epoxy resin, while the fibers obtained by the present method can be mixed with the resin at a rate of more than percent, whichdifference of which is recognized to be remarkable.

EXAMPLE 2 The following eight kinds of raw material was used for manufacturing carbon fibers under the conditions as shown in Table 3 below.

Raw Material Preparation P-l Obtained by heating bottom oil resulting from naphtha cracking for ethylene production at a temperature of 360 C under presure of 10 mrnI-Ig to low boiling point fractions therein Obtained by heating tar resulting from a high temperature naphtha cracking at 380 C under 10 mmI-Ig to remove low boiling point fractions therein Obtained by first making heavy anthracene oil available in the general market through reaction with air at 200 C in the presence of 5% by weight of solid ammonium nitrate, and then removing the low boiling point fractions at 350 C under 10 mmH Obtained by removing low boiling point fractions of petroleum pitch at 380 C under 10 mmHg Obtained by treating coal pitch with chloroform to extract and remove low molecular weight substances, after which the coal pitch is heated at l50 C for 3 hours, then at 200 C for 3 hours, and further at 300 C for 1 hour under a reduced pressure Obtained by treating distilled bottom oil which is produced by partial hydrogenation of tar from cracking of crude petroleum in an atmosphere of air containing l0% by volume of NO, at a temperature of I00 C for 1 hour, then treating the oil in ammonia gas at 200 C for 1 hour to make the same heavy, and finally removing low boiling point fractions at 350 C under 10 mml-Ig Obtained by pyrolysis of polyvinyl chloride at 405 C in an inactive gas current Obtained by heating 45 parts by weight of phenanthrene and 55 parts by weight of chrysene in the presence of 10% by weight of aluminum chloride catalyst, and then refining the same Physical properties of carbon fibers thick- Elonganess, tion Strength Orienrnicron percent (tJcmJ) tation 9. 2. 6 8. 0 None.

in o is applied to filament yarn and staple yarn, respectively. The Experiments Nos. 7, 8, and 9 are examples of the deformation of the is varied. The

I TABLE 3 Infusibilization Carbonization Final Thread Tempertreating ature Time Atmostemp. Atmosphere 0.) (hr.) phere 0.)

1,000 (a 140 .....do Ch 0.6 Hz 800 210 .....do.. 0 1 5510113 280 2 NH; 150

0 150 .do Air pl usSO; 100 0.5 Ar 1,000

170 H0104 plus 30 1.0 Ar 1,500

H20 (10%). 210 do... NOCl 0.5 N: 950

2. Carbonization time: 6 hours for each run.

and partly cross-section.

The Experiment Nos. 1, 2, and 3 indicate the results of varying the ratio of strength of the twisted yarn within the limit established by the present invention.

sted and graphitized in 2 5 The Experiments Nos. 5 and 6 show the test results,

which the minimum stretch rati fiber cross-section when the stretch ratio TABLE 4 Raw material Mean Soften molecular ing C/C+H, weight point Spinning (MW) 0.) method 95.2 590 230 Melt- Air spinning. 95.0 530 .do....... Air plus 0 Nora:

l. Spinning method: A melt-spinning, wherein centrifugal force and high speed hot air current are used together, is adopted. The spinning speed is 2,000 m./min., and the draft ratio is 5,000 times.

Carbon fibers obtained from each raw material in the These carbon fibers were t Kind percent P-ZA.

manner as shown in Table 3 contain more than 90 percent of carbon, and assume a substantially circular cross-sectional shape, or, oval in some parts.

accordance with the present invention, the results of which are shown in Table 4 below. As is evident from the Table, the graphite fibers obtained by the present method are recognized to be excellent.

L w. m 8 006 w d d S fl m m WM W mmm mm. Tam 1 r s m n n PM Wm P m a m m m m m r n o fi E b m. m o B m l H n w t r. o n n n a madman twmaadaa m m d m 0 m 6 e W mm m m mnnwnn nnnnn m m 6 h m m H WC M39 M 0 no. m cm a D. m d m %m 3'32 amen mama mm 0 a e n w m P 1 m 11 d g e e n mic 444: 123 mye X t a a I. mow m ma m 0. m m mnpm m mm mm m 50 wwwnmw mmmnwwm m 8 m h m e 3 t m m hh m mum 0 0 0 0 0 1 0 0 0000 m M W t t m m m m H me P m W ma W 4 e O m me n r n 3 m m m m a m mm. m 1 3 .4 wi h; w w h .D f X e a. fi mm mmnmw mmmmmm mm 1 C a 0 e pflv W W m 0M .0. m mW sw H n H mm" .222 ll fi l b 00 H mm innoo Hamlin m m a m w y m D. nn mm. mm" n m eh m 5 m r m 22 mm d S tt m mwwmmw wwmmmmm mwm M m 8 m a E a W mmm mum new w t e d f T. n mmm .mm mw lmf n muse mm C m r. O n 3 .l 5 C an 3 3M mm e n o P s w e m E m -3... h m N e .m m o L m .0 t m n B m m 011 mw y or. T w I J n m mmmmnm WWWWWWW E h m b m .m a a T( hwc w 0 5 mm 5 5 0 www mm mwm mw t m un 0000 0000410 s n c X m o g n a a h m em 29. .ma nman .mm mm T m .m C m t I J wr II a U was an. m m b a .w .m n .w m n z. m .n d m w n .m m n .0 d w m 5obc m I m 33? 30 t n e PE t m n t. m 5 13 050 aaawass um 6 h n a n n m e m W e gmsb n U m We amn o Hamw. w 1m x a w 5 m ma w u 0 m m a 5 05 117 W f r m h b n r n H .w T mm o aek Wfi mw In m m n m m m m a m 0 m m 0 when a o ii Pm n .Qm 0 mm .Wfifi c m w e r .w w antenn o n w e a a m m a t r m u m v 4 3 gm m. e I wm m W k m e a m S m MAM BB A 9% P o t b w 8.... o i c m e S n O C F .F o iii 4 3 .4 an... e r a y e n w mN PPPPPP PPP PPP m R T u m m m .W m a u n w 7 e c t r a s 106 mm 3mr.nitF h h so u. on mwhflnm mwnnnnn w mmmm d d h w w m W W m m EN 1 2 3 m m e .l f wwmmm m mo m wm vwn um N mdmsommb sw mm hm m mw mmmm wmu mi m liaise zsimnmu m a 0 D. C D. m w m N m D. a

Upon completion of the first step heating, the graphite fibers of Ex. Nos.1 and 2 showed a strength of about 14 T/cm and 18 T/cm respectively, and elongation of less than 0.9%, although the cross-section remained nearly a true circle.

The graphite fibers were then twisted and subjected to the second step heating. Ex. No. 3 shows a result, in whichthe graphite fibers after the first step heating were twisted and further heated at a temperature higher than the first heating in the twisted state. The physical properties of the graphite fibers after the second step heating are shown in the following Table 6.

The structure of the filament yarns used in this example are such .that vabout 300 monofilaments are subjected to the first-twisting, and then three-sets of the After the second step heat-treatment, the graphite yarn of Ex. No. 3 was loosened in such a manner that its twists are divided into the initial three sets. It was ob served that the yarn was heat-set in wavy form, though having a very gentle wave. When this yarn was again heat-treated at 3,000 C. applying a very small tension thereto, it returned to a straight form in appearance, although the filaments in the yarn still retained the their twisted appearance.

What we claim is:

1. A method for the production of graphite fibers of a circular cross-section which comprises the steps of:

' twisting carbon fibers having a strength of 7 T/cm and above at a ratio strength of from 0.03 to 0.5 times that of the original fibers with respect to the apparent rupture strength of the twisted yarn; and heat-treating the twisted yarn at a temperature of at least 2,000 C. in an inactive atmosphere, while imparting a tensile stress of at least 0.5 'T/cm to said twisted yarn, after the twisted yarn is brought into the state of being able to convert the tensile stress into a compressive force.

2. A method according to claim 1, in which the tensile stress is above 0.7 T/cm 3. A method according to claim 1, in which the tensile stress is above 2,500 C. 

2. A method according to claim 1, in which the tensile stress is above 0.7 T/cm2.
 3. A method according to claim 1, in which the tensile stress is above 2,500* C. 